Such measuring cells have, for example, a sending coil embodied as a toroidal coil, which is fed by an alternating voltage. In the interior of the sending coil, an annular, magnetic, alternating field is produced. In the same plane in which also the sending coil lies, a receiver coil is arranged, which likewise can be embodied as a toroidal coil. Through movable ions in the liquid, measured material, as a result of the magnetic alternating field in the sending coil, a ring shaped electrical current is produced in the measured material, which, in turn, causes in the receiver coil an output signal, whose strength is dependent on the mobility and concentration of the ions and, consequently, on the electrical conductivity of the liquid, measured material. The output signal is usually in the form of an induced current.
The principle is, per se, established in industrial process measurements technology and discussed in a number of documents of the patents literature, examples of which include U.S. Pat. No. 3,603,873 and German Offenlegungsschrift (Laid-Open Application) DE 198 51 146 A1.
For Ex-protection reasons, a resistor RA can be inserted in the secondary circuit. If the output signal is tapped as voltage UInd from the receiver coil, the following relationship holds:
            U      Ind        =                            R          A                ·                  I          Ind                    =                                    N            1                                N            2                          ⁢                  1                      1            +                                          N                2                            ·                              R                Med                            ·                              (                                                      1                                          R                      A                                                        +                                      1                                          jω                      ⁢                                                                                          ⁢                                              L                        2                                                                                            )                                                    ⁢                  U          Ex                      ,where N1 is the number of turns of the sending coil and N2 the number of turns of the receiver coil. RMed refers to the resistance of the measured material, j is the imaginary unit and ω the angular frequency of 2πf, wherein f is the frequency of the induced alternating current. Furthermore, UEx refers to the exciter voltage and IInd the electrical current induced in the receiver coil of inductance L2.
In case RA<<ωL2, then the inductance L2 of the secondary coil has a negligible influence on the measured value.
If RA≦≦ωL2 is no longer true, then the measured value becomes dependent on the inductance L2 of the receiver coil. This inductance L2, in turn, depends on the core permeability, which is a function of temperature. Therewith, the measured value is temperature dependent (order of magnitude: About 1% per 100° C.). In order to compensate for this, it is important to determine the inductance of the receiver coil. For example, especially in the case of sterilization procedures, where the conductivity-measuring cell experiences, up to 30 times daily, temperature fluctuations between 5° and 150° C., a diagnosis of the sending- and/or receiver coil and an associated adjusting of the measured value is required in accordance therewith.
From U.S. Pat. No. 6,414,493 B1 and German Offenlegungsschrift (Laid-Open Application) DE 4116468 A1, as well as also from German Patent DE 1028679, it is known to provide an additional conductor loop, which passes through the sending coil and through the receiver coil and serves as an additional, short-circuit path. For diagnostic purposes, the conductor loop is closed and an electrical signal induced in the receiver coil, or in the secondary circuit, and ascertained by means of an evaluation unit connected to the receiver coil, or to the secondary circuit. From the induced output signal, a state of the receiver coil, especially damage to such, can be ascertained.
Moreover, it is known from European Offenlegungsschrift (Laid-Open Application) EP 0999441 A1, for monitoring the sending coil of inductive conductivity-measuring cells, to determine the inductance by means of electrical current, or voltage, measurement. The receiver coil of the secondary circuit can, however, not be reliably monitored therewith.